The present invention relates to a display apparatus comprising a colour display tube including a penetron cathodoluminescent screen and a power supply for switching the post deflection acceleration (PDA) voltage between different values in order to produce the desired colour.
Penetron screens are known and are discussed in an article "Performance of Penetration Colour CRTs in Single-Anode and Dual-Anode Configurations" by G. R. Spencer in Proceedings of the SID Vol. 22/1, 1981, pages 15 to 17. G. R. Spencer highlights some problems in using penetron screens in single anode cathode ray tubes. As is known, different colours are produced using a dual primary-colour penetron phosphor by varying the anode to screen voltages of the tube. One effect illustrated in broken lines in FIG. 3 of the Spencer article is that the spot size and thus the line width changes over the range of voltages that can be used. Accordingly the electron beam has to be refocused if the spot size is to be maintained constant. Another problem with varying the anode to screen voltages is that in order to maintain a substantially constant picture size then the deflection current has to be varied with screen voltage. G. R. Spencer proposes reducing the effects of these problems by separating the anode of the electron gun and the transparent electrode on the phosphor screen into two independent electrodes. However this dual electrode arrangement produces an increase in line width with increasing beam current and requires an increase in deflection current for increases in screen voltage.
One proposal for separating the addressing of an electron beam from the light and colour generation in a display tube employing a penetron screen is disclosed in British Patent Application No. 8230244. This patent specification discloses a single beam display tube comprising a channel plate electron multiplier which comprises a stack of apertured dynodes the holes in which are aligned to form channels. A low energy electron beam is scanned across the input face of the electron multiplier. The electron multiplier produces a current multiplied electron beam which is used for light and colour generation. The cathodoluminescent screen comprises groups of phosphor elements, at least one phosphor element of each group comprising a penetron component with two colour phosphors such as red and green, and another phosphor element having a different phosphor such as blue.
Another approach to producing coloured images from a display tube including a channel plate electron multiplier and a penetron cathodoluminescent screen is disclosed in British Patent Specification No. 1402547. In Specification No. 1402547 a continuous two-layer red-green penetron phosphor layer is provided on the faceplate or other optically transparent carrier substrate disposed between the output surface of the electron multiplier and the faceplate. Additionally a blue light emitting phosphor is provided on a first colour selection electrode carried by the output surface of the electron multiplier and a second colour selection electrode is provided between the green penetron phosphor and the faceplate or its supporting substrate, the red penetron phosphor being closer to the electron multiplier than the green one. In operation, by varying the field set up between the first and second colour selection electrodes one of the different phosphors can be activated by the electron beam emerging from the channel plate electron multiplier.
When scanning such screens it is necessary to switch rapidly the PDA voltage applied to the screen electrode, which is coextensive with the cathodoluminescent screen, between a low voltage of, for example +7 kV relative to the output of the channel plate electron multiplier and a high voltage of, for example +14 kV; otherwise the colour purity of the image will be adversely affected.
Known hig voltage circuits frequently comprise a low voltage source which is stepped-up to a high voltage by a transformer and rectifier arrangement, or a voltage multiplier of, for example, Cockroft-Walton type. Generally a capacitor is connected between the output terminal of the high voltage circuit and a reference voltage point, for example ground. In operation the voltage at the output terminal can be increased rapidly, but the reverse is not always the case because of the time taken for the charge on the capacitor to decrease to its desired level. This problem is generally overcome by discharging the capacitor by suitable switching means such as a thermionic valve or a series combination of high voltage transistors. In both cases the additional circuitry is bulky and/or expensive, especially if used with a flat display tube of the type disclosed in British Patent Specification 2101396 having a screen size of the order of 100 mm diagonal and using a high resolution glass matrix electron multiplier.